1. Field of the Invention
The present invention relates to a molded lens used in an optical system of an optical image recording apparatus such as a camera, a facsimile machine, a laser beam printer or a copying machine. More particularly, the invention relates to a plastic lens used as a scanning optical element mounted in such an image forming apparatus.
2. Description of the Related Art
For the plastic lens used in an optical system of an optical image recording apparatus, there is a demand for a lower birefringence and a lower GI (refractive index distribution). In an optical system of a laser beam printer which is an optical image recording apparatus, for example, birefringence of a plastic lens causes a bulge of the laser beam. As shown in FIG. 11, in a plastic lens, GI (refractive index distribution) is such that the refractive index is higher on the outer periphery than in the interior. The lens has therefore a negative power as shown in FIG. 12. Internal strain in the plastic lens is released as a result of secular change, and this causes release of GI (refractive index distribution), thus resulting in a displacement of focus.
FIG. 11 schematically illustrates a gradient of refractive index caused in an fxcex8 lens as a result of a short molding time. The lens outer periphery in close contact with the mold while being formed is first cooled and solidified. The lens outer periphery has therefore a higher density than the interior of the lens. The refractive index of the lens is accordingly higher on the lens outer periphery than in the lens interior. The solid line in the lens shown in FIG. 11 represents an isoflux of the refractive index, suggesting that the refractive index is higher toward the lens outer periphery.
The above-mentioned displacement of focus results in deterioration of birefringence of a printed image of a laser printer. As counter-measures against this inconvenience, methods for reducing birefringence are conventionally proposed by Japanese Patent Laid-Open Nos. 11-19956 and 10-278077 (USSA 2001028134) and Japanese Patent 2001-62870. Methods for reducing GI (refractive index distribution) are proposed in Japanese Patent Laid-Open Nos. 8-15503 and 09-49976 (U.S. Pat. No. 5,966,161).
The following three means for reducing birefringence by producing a sink mark in a non-effective area of the plastic lens are proposed:
(1) Causing a sink mark by local heating by a heater during the cooling step;
(2) Causing a sink mark by forcedly releasing the resin from the mold by injecting air during the cooling step; and
(3) Causing a sink mark by forcedly releasing the resin from the mold by pulling out the mold core during the cooling step.
In the case of (1) above, however, a problem is encountered in that embedding the heater in the mold leads to a complicated mold structure. The case (2) requires an air channel in an air blower or the mold, resulting in complicated apparatus and operation. This method is low in forming stability since the amount of air injection or pressure leads to deterioration of the surface accuracy.
The case (3) requires a special mold core structure or a driving source such as an oil hydraulic cylinder for causing sliding of the mold core, resulting in complicated apparatus and operation.
As means for reducing GI (refractive index distribution), the portion having a high GI (refractive index distribution) is kept away from the flux effective area by providing a convex portion in the non-effective area of the plastic lens. In order to obtain this advantage, it is necessary to enlarge the convex portion relative to the thickness of the plastic lens in general, and the forming cycle cannot be reduced. This causes a problem in that the lens itself must be larger in size, thus leading to a higher cost.
It is therefore an object of the present invention to provide a plastic lens which permits reduction of the forming cycle and has reduced birefringence and GI (refractive index distribution).
To achieve the above-mentioned object, in a molded lens, such as a plastic lens of the present invention, a concave shape is provided at least in a part of a non-effective area through which an image forming flux does not pass.
According to the above-mentioned configuration, forming under a reduced pressure condition causes accumulation of heat in the concave portion in the non-effective area during the cooling step. This in turn causes a delay in cooling, thereby making it possible to selectively produce a sink mark. A sufficient transferability of the optical surface can therefore be ensured.
In portions other than the concave portion, a portion with a high GI (refractive index distribution) can be kept away from the flux effective area. It is therefore possible to inhibit deterioration of GI (refractive index distribution).
If the plastic lens has a center thickness d and a lens height h at the concave portion, achievement of a shape satisfying d greater than h permits reduction of the forming cycle because of the cooling of the concave portion.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.